Sedimental Journey:Fossils in the Context of Geological Time

Stratigraphy is the scientific study of geological history. It originated about 200 years ago with the work
of an English engineer named William Smith. Smith saw the same layers of fossil-bearing rock as he surveyed
different parts of England. He drafted stratigraphic maps from which he could predict accurately the location of
undiscovered coal beds, because he knew which rocks overlay known coal beds elsewhere in England.

He deduced two important principles of stratigraphy:

each strata of rock has a characteristic assemblage of fossils, which makes it possible to relate similar strata in different places;

younger strata are superimposed on older strata.

These principles enable geologists to date rock layers and the fossils they contain relative to other layers.
This relative dating provided geologists with the basis for the scientific division of the history of life on earth.
Later, the discovery of radioactivity in the 20th century gave scientists tools to date rocks absolutely and to refine
these divisions.

In this section, we are going to define the major divisions of geological time, starting with the most general divisions
and working towards the more specific subdivisions.